Sir

You report that scientific papers from the United Kingdom have been cited on average 4.19 times but that those from Japan notch up only 3.18 citations (Nature 387 537; 537 1997). Do these relative scores reflect more than the reading (and non-reading) habits of Americans, who themselves dominate the writing (and citing) of scientific literature?

A paper's ‘impact factor’ measures, as much as anything else, how visible and accessible it is in the United States. It would be misleading and unfair to our Japanese colleagues to infer a genuine ‘quality gap’ from bibliographic data of this kind.